A key limitation in many data processing circuits is the number of data output signals which are available. This is especially true in the case of integrated circuits, where the number of circuit elements vastly outnumbers the available output pins. Accordingly, it is desirable to send as much data as possible between two points by using the minimum number of interconnects.
In a traditional "single-ended" data signaling method, a single data signal is sent over one data channel, such as a wire, by varying a signal attribute, such as current or voltage. For example, in a digital data transmission, sending a 0 volt signal may indicate a digital zero bit, while sending a 5 volt signal indicates a digital one bit. This conventional method of data signaling is adequate if the signal levels are widely spaced and well defined. However, power consumption by electronic devices has recently become a major issue. To address this concern, the supply voltages and the separation between different data levels have been significantly reduced. Separations in the range of only several hundred millivolts are not uncommon. Unfortunately, single-ended data transfers are relatively susceptible to noise and when signal swings are reduced, noise becomes a serious issue and even small amounts of interference can seriously degrade the reliability of the interface.
A conventional solution to line noise has been to use a differential mode of signaling. A single data signal is transmitted over two wires, each of which carries one signal component. The two components are generally derived from the same source data signal and are varied such that the data signal is transmitted as the difference between the signal level of the two signal component.
In digital environments, differential data signals are transmitted as two voltage signals of opposite polarity relative to a reference level (differential voltage signaling). The transmitted data is extracted by determining which signal component has a greater voltage. By changing the voltage polarity of the signal components, the desired data can be transmitted. Alternatively, current signaling may be used, in which a differential signal is represented as two current signals flowing in opposite directions on a closed loop. The direction of current flow indicates the polarity of the digital signal transmitted. By changing the relative polarity of the voltage signal components direction of current flow, the desired data may be transmitted.
Differential mode signaling provides for greatly improved noise immunity, lower power, as well as less noise generation and is therefore widely used to interconnect digital circuits on separate chips and circuit boards. However, a significant problem with differential signaling is that two wires are required to transmit one data signal. This is a particularly serious issue where integrated circuits are concerned because the number of input and output pins available is extremely limited. It is therefore desirable to increase the amount of data which can be transmitted over a digital data interface, while retaining the power and noise immunity advantages associated with two-wire differential signaling. It is also desirable to transmit additional information over a two-wire differential interface without decreasing the accuracy of the original differential mode signal and without increasing the number of interface wires required.
Differential signaling has also been used in the telecommunications environment, specifically in the context of two-wire "twisted-pair" audio communication. Transmitting and receiving circuits are coupled to the interface using transformers. In the context of audio communication over telephone cables, the coupling transformers have been center-tapped to allow an additional signal to be transmitted on the twisted pair. Two center-tapped circuits have been combined using transformers as shown in FIG. 1 to create a "phantom circuit." As shown, two analog signals are transmitted in a normal manner over each of the balanced pairs. A third analog signal is transmitted over the four wires through the center taps of the transformers. Provided that all four of the wires are precisely balanced, the first two signals are not affected by currents entering and leaving through the center taps of the transformer windings.
A significant drawback to this type of circuit is the limiting nature of the transformer coupling. Transformers are relatively narrow band-pass structures and the attributes of the transformers used in a specific application must be chosen according to a predetermined, and limited, input signal bandwidth. For standard telephony, this limit is 300 Hz to 3300 Hz. Signals with a frequency outside of the designed bandwidth are attenuated by the transformer interface and not passed by the system. Thus, while phantom circuits may be suitable for transmitting a third, limited bandwidth signal over two narrow band voice signals, transformer based circuits are unsuitable for broad-band or variable-band data communication. In particular, they are unsuitable for digital communication because the pseudo-random nature of digital signals results in signal frequencies which can range from zero Hertz, for a string of bits with the same polarity, to several giga-Hertz, depending on the data content. An additional drawback to this type of circuit arrangement is the difficulty of maintaining a balanced interface when more than a few twisted pairs are present. This is especially difficult because interface characteristics vary across the length of a telephone cable due to shifting in the relative position of one pair of wires with respect to others, and the tapping of various lines as additional subscribers are connected.